This chapter introduces the technical vocabulary used for computer audio hardware.

1.1. Types of Sound Cards

A sound card is a hardware device which allows a computer to process sound. Most sound cards are either audio interfaces or MIDI interfaces. These two kinds of interfaces are described below.

1.1.1. Audio Interfaces

An audio interface is a hardware device that provides a connection between your computer and audio equipment, including microphones and speakers. Audio interfaces usually convert audio signals between analog and digital formats: signals entering the computer are passed through an analog-to-digital convertor, and signals leaving the computer are passed through a digital-to-analog convertor. Some audio interfaces have digital input and output ports, which means that other devices perform the conversion between analog and digital signal formats.

The conversion between analog and digital audio signal formats is the primary function of audio interfaces. Real sound has an infinite range of pitch, volume, and durational possibilities. Computers cannot process infinite information, and require sound to be converted to a digital format. Digital sound signals have a limited range of pitch, volume, and durational possibilities. High-quality analog-to-digital and digital-to-analog convertors change the signal format in a way that keeps the original, analog signal as closely as possible. These quality of the convertors is very important in determining the quality of an audio interface.

1.1.2. MIDI Interfaces

Musical Instrument Digital Interface (MIDI) is a standard used to control digital musical devices. Many people associate the term with low-quality imitations of acoustic instruments. This is unfortunate, because MIDI signals themselves do not have a sound. MIDI signals are instructions to control devices: they tell a synthesizer when to start and stop a note, how long the note should be, and what pitch it should have. The synthesizer follows these instructions and creates an audio signal. Many MIDI-controlled synthesizers are low-quality imitations of acoustic instruments, but many are high-quality imitations. MIDI-powered devices are used in many mainstream and non-mainstream musical situations, and can be nearly indistinguishable from actual acoustic instruments. MIDI interfaces only transmit MIDI signals, not audio signals. Some audio interfaces have built-in MIDI interfaces, allowing both interfaces to share the same physical device.

In order to create sound from MIDI signals, you need a "MIDI synthesizer." Some MIDI synthesizers have dedicated hardware, and some use only software. A software-only MIDI synthesizer, based on SoundFont technology, is discussed in Chapter 10, FluidSynth

You can use MIDI signals, synthesizers, and applications without a hardware-based MIDI interface. All of the MIDI-capable applications in the Musicians' Guide work well with software-based MIDI solutions, and are also compatible with hardware-based MIDI devices.